There has been an ongoing desire for capacitors with improved characteristics. Of particular interest is a capacitor with a decrease in parasitic resistance. Various components of the capacitor have been examined, modified and improved towards this end. The present invention is primarily directed to the lead wire configuration and improvements, both specific and synergistic, related thereto.
Resistance in lead wires is generally defined by the equation for resistance which is:Resistance=resistivity×path length/cross-sectional area.
Based on this relationship increasing the cross sectional area available for current flow reduces the resistance in the wire. This leads one of skill in the art towards as large an anode lead wire as possible with the theoretical limit being the physical size of the anode and package. Due to physical size limitations the maximum cross-sectional area for current flow through a single cylindrical lead wire is nd2/4 wherein d is the diameter.
Increasing the lead wire diameter decreases resistance in the wire itself and between the wire an anode. Unfortunately, increasing the wire diameter also reduces the capacitance by displacing porous anode body material which would otherwise contribute to capacitance. The use of multiple lead wires reduces the path length for current to flow through the porous anode body. A porous anode body has a high resistance relative to the lead wire. Therefore, with multiple anode lead wires the path length from points in the porous body are closer to at least one anode lead wire when multiple lead wires are used. This has led to the use of multiple anode leads as described in U.S. Pat. No. 7,116,548 which is incorporated herein by reference.
In addition to the desire for decreased internal parasitic resistances there is a continual desire to decrease the cost of capacitors. There are multiple components to decreasing cost including decreasing waste material and decreasing faulty product. Multiple, or large, anode lead wires are a detriment to both aspects. The use of multiple anode wires is a significant contributor to manufacturing inefficiency and increased capacitor cost.
The electrochemical processes used to manufacture solid electrolytic capacitors typically involve multiple dipping operations. In order to minimize the variation in dip depth, as measured by percentage of the anode length, the lead wire is typically inserted parallel to the longest dimension of the anode body which is also referred to as the length of the anode. During dipping the lead wire is used as a handle for attaching the anode to manufacturing equipment. After dipping most of the lead wire is removed during the assembly process. This creates a significant amount of wasted anode wire material which is known to be a particularly expensive material. Decreasing the diameter of the lead wire decreases waste but at the expense of ESR as described above.
Yet another problem is the defects which occur as a result of cutting the lead wire. The cutting operation has been determined to be a contributor to current leakage due to the stress created at the intersection of the lead wire and anode body. Smaller diameter lead wires mitigate the leakage caused during trimming of the lead wire.
In addition to a decrease in parasitic resistances and manufacturing cost there is an ongoing desire for improved volumetric efficiency. Volumetric efficiency of a capacitor is defined as the capacitance per unit volume. One approach to maximizing volumetric efficiency is to maximize the volume of the package filled by the anode, dielectric and cathode while minimizing the space occupied by terminations and encapsulating materials. Packaging efficiency is defined as the ratio of the volume occupied by the anode, dielectric and cathode to the volume occupied by the finished capacitor which includes anode termination and cathode termination. In order to minimize the volume required for the cathode termination the termination is best made from the bottom of the capacitor as it sits on the circuit board. In order to further minimize ESL both the anode and cathode terminations should be made from the bottom of the capacitor as it sits on a circuit board. Typically, the anode wire used for handling the anode during electrochemical processing, is parallel to the circuit board which precludes mounting the capacitor with both the anode and cathode in direct contact with a substrate. Direct contact with the substrate describes a mounting technique wherein the anode lead wire, and/or cathode, are attached to a substrate directly without a lead frame there between. In this instance the anode lead wire, and/or cathode, is soldered directly to the conductive trace of the substrate.
The present invention provides a method of manufacturing a capacitor, and capacitor manufactured thereby, wherein the ESR and ESL are improved. In addition to the improved properties the capacitor can be manufactured at a reduced manufacturing cost due to a decrease in waste and a decrease in defective product. In addition, the manufacturing method allows formation of a capacitor with an increased volumetric efficiency.